The motherboard information should tell you if you are able to use DDR or SDRAM . Some motherboards will allow both, but you cannot mix and match, its either DDR or SDRAM .
Also , you may be able you look on the board itself. The slots amy say DDR or SDRAM beside them.
Registered? Do you mean buffered or unbuffered? You want unbuffered. Also, you need to make sure the pin-count matches, not to mention the speed of the RAM has to match your processor. Whether that be 2600, 3200, etc. If your manual doesn't say one way or another, you DEFINATELY need to call the manufacturer and ask them. Installing incorrect RAM is a big no no.
If your motherboard doesnt support Registerd DDR chips then they will not work. Same if you try to run regular DDR chips in a motherboard that required Registered it will not even post. As Hanson pointed out usually the only boards that need Registered are high end server boards. Check with your motherboard manufacture to see what your motherboard requires.